Dr Moh Malek, from Wayne State University in Detroit, who led the US study on mice, said: "Mitochondria produce energy which is used by the cells in the body. More mitochondria mean more energy is produced, the more work can be performed.

"Aerobic exercise, such as running or cycling, is known to increase the number of mitochondria in muscle cells. Our study has found that epicatechin seems to bring about the same response - particularly in the heart and skeletal muscles."

A specific type of epicatechin from cocoa was given to mice twice a day for 15 days.

At the same time, the animals underwent 30 minutes of treadmill training each day.

Researchers found that mice only fed epicatechin had the same exercise performance as those running on the treadmill.

The scientists hope their research,Â published in the Journal of Physiology, will lead to better ways of combating age-related muscle wasting.